World War 1 Begins

On July 28, 1914, one month to the day after Archduke Franz Ferdinand of Austria and his wife were killed by a Serbian nationalist in Sarajevo, Austria-Hungary declares war on Serbia, effectively beginning the First World War.
Threatened by Serbian ambition in the tumultuous Balkans region of Europe, Austria-Hungary determined that the proper response to the assassinations was to prepare for a possible military invasion of Serbia. After securing the unconditional support of its powerful ally, Germany, Austria-Hungary presented Serbia with a rigid ultimatum on July 23, 1914, demanding, among other things, that all anti-Austrian propaganda within Serbia be suppressed, and that Austria-Hungary be allowed to conduct its own investigation into the archduke's killing. Though Serbia effectively accepted all of Austria's demands except for one, the Austrian government broke diplomatic relations with the other country on July 25 and went ahead with military preparedness measures. Meanwhile, alerted to the impending crisis, Russia—Serbia's own mighty supporter in the Balkans—began its own initial steps towards military mobilization against Austria.